Method and apparatus for aircraft pitch and thrust axes control

ABSTRACT

An aircraft automatic or semiautomatic vertical path control system which coordinates operation of pitch and engine thrust control systems to transfer speed control from one system to the other depending on a requirement to climb, descend, or maintain altitude as determined by the polarity and magnitude of the difference between a selectable desired altitude and current actual altitude.

This invention relates to vertical flight path control of an aircraftand more particularly to method and apparatus for achieving throughautomatic or semiautomatic control, single mode selection for bothautopilot and autothrottle.

Improved speed control of an aircraft which is being automaticallycontrolled simultaneously in the pitch axis by elevator control and inthe longitudinal axis by throttle control is exemplified by U.S. Pat.No. 2,888,219 to Beers, et al. issued May 26, 1959. Beers, et al. isillustrative of the use of a pitch command rate input signal into thethrottle control law to automatically compensate for speed changes dueto aircraft pitch attitude changes. In contrast, a preferred embodimentof the present invention relates to a means for selecting theappropriate modes of operation for the elevator and throttle systems toprovide automatic climb, descent, and altitude capture and hold througha common set of select controls without special emphasis on the specificcontrol laws or signal sources required to implement such control lawsused in the various modes of operation.

An early U.S. Pat. No. 2,961,200 to Seliger, et al. issued Nov. 22, 1960relates to an automatic altitude hold and speed hold system forpropeller driven aircraft. Seliger, et al. discloses the physicalimplementation (sources required) for acquiring and processing therequired signals. In contrast, a preferred embodiment of the presentinvention is not concerned primarily with the signal acquisition andprocessing of control laws but with logic control of operational modesof aircraft elevator and thrust systems.

U.S. Pat. No. 2,933,268 issued Apr. 19, 1960 to Jude, et al. shows asystem for providing stall prevention for an aircraft which is beingautomatically controlled in the pitch (elevator) and longitudinal(thrust) axis. In contrast, Jude, et al. is further directed toautomatic compensation for aircraft configuration changes (gross weight,flap position, etc.) and the effects of wind gusts.

A further exemplary reference in the field of altitude and thrustcontrol systems includes U.S. Pat. No. 3,945,593 to Schanzer issued Mar.23, 1976. Schanzer shows automatic control of elevator and throttles ofan aircraft in a manner for reducing coupling between pitch andlongitudinal axis with emphasis on control laws and aircraft statevariables to implement the control laws.

Turning now more specifically to aircraft vertical path control aspresently practiced, it should be noted that during the climb anddescent phases of a flight, an aircraft is commonly controlled in speedthrough the elevator system and in rate of climb and descent throughengine thrust. However, during the cruise phase or whenever a particularaltitude must be maintained, the altitude of the aircraft is controlledthrough the elevator and the speed is controlled through engine thrust.

Desired speeds and rates of climb or descent are variables which aredependent on many factors. For example, the climb speed may beestablished initially as an indicated airspeed (knots) for the firstportion of the climb and subsequently established as a desired machnumber; with both values being determined to provide, e.g. minimum fuelto altitude, or minimum time to altitude. Similarly, the engine thrustsetting is commonly established for maximum fuel conservation, or, forother considerations. Also, during cruise, the altitude and speed (orMach number) are variable and chosen to provide, for example, maximumrange.

When a pilot is manually flying the aircrfat, he is required to controlthe aircraft elevator and engine thrust, through the manual controls, toestablish and maintain the desired values of speed, altitude, rate ofclimb, etc., as indicated to him by the various aircraft instruments.Prior automatic and semiautomatic flight control systems have so farprovided automatic, or semiautomatic modes of control in one or both ofthe control axes.

Such devices as engine thrust (EPR or N₁) control and airspeed and MachHold control modes for the elevator have been utilized, and in theutilization of these devices, the pilot selects the appropriate mode ofoperation for the pitch (elevator control) axis and for the enginecontrol.

According to present flight control practices it is possible to haveboth control systems engaged or only one control system engaged andoperate the other manually. However, apart from interlock devices whichprevent both systems from trying to control the aircraft speed at thesame time, the two control systems are independent.

Therefore, in the aforementioned systems, the pilot must manually effectthe change over from one device controlling speed to the other. Forexample, the pilot may engage the thrust control system to maintain adesired engine speed (N₁) or Engine Pressure Ratio (EPR) and engage thepitch (elevator) control system to maintain the desired indicatedairspeed (IAS) or Mach number for climb. If now, a given altitude mustbe maintained, the pilot must engage the pitch axis in an altitudecontrol mode and change the thrust control to maintain aircraft speed orMach number. If then, subsequently, the pilot wishes to resume climbinghe must change the operating modes of both the pitch and thrust controlsystem to establish the desired flight path. Furthermore, these actionsoften necessitate the manipulation of speed select control devicesand/or the precise operation, in time, of speed control modes if thecontrol mode is of the hold type rather than the select type.

Accordingly, it is an object of this invention to provide a single speed(IAS or Mach) control device which provides the desired reference valuewhether the pitch axis control system or the engine thrust controlsystem is controlling the aircraft speed.

It is another object of the invention to provide a means of selectingand displaying a desired altitude and an automatic control to captureand maintain that altitude through the pitch control system.

It is a further object of the invention to provide means forautomatically capturing and maintaining the present altitude.

It is yet another object of the present invention to provide a methodfor coordinated operation of pitch and engine thrust control systems ofan aircraft to automatically transfer speed control from one system tothe other depending upon current requirements; i.e., climbing,descending, or maintaining altitude.

It is a still further object of the invention to provide system couplingthrough a further device containing preprogrammed or pilot programmablevertical path profiles to generate the values, either fixed or variable,for aircraft speed, engine thrust, and altitude.

It is still another object of the invention to provide means forcoupling and uncoupling the computed control outputs to either theautomatic elevator and engine control systems or to couple the pitchcontrol outputs to a Flight Director for semiautomatic operation.

It is a still further object of the invention to provide the currentmaneuver requirements, i.e. to climb, to descend, or to maintainaltitude by means of a device which determines the polarity andmagnitude of the difference between a selectable desired altitude andcurrent actual altitude.

These and other objects are achieved in accordance with a preferredsystem embodiment of this invention which includes a single speedcontrol device and readout which provides the desired reference valuewhether the pitch axis control system, or the engine thrust controlsystem is controlling the aircraft speed, a mean for selecting anddisplaying a desired altitude and an automatic control to capture andmaintain that altitude through the pitch control system, a means forautomatically capturing and maintaining the present altitude, acoordinated operation of the pitch and engine thrust control systems toautomatically transfer speed control from one system to the otherdepending on whether the aircraft is commanded to climb, descend, ormaintain altitude, a logic system based on the polarity and magnitudedifference between the selected altitude and present altitude fortransferring aircraft speed control automatically from the engine thrustcontrol system to the pitch control system and also automaticallyestablishing the correct engine control operation depending upon whetherthe pilot wishes to climb or descend, means for the system to beconnected to another device which contains preprogrammed or pilotprogrammable vertical path profiles to generate the values either fixedor variable for aircraft speed, engine thrust and altitude, and means tocouple and uncouple the computed control outputs to either the automaticelevator and engine control systems or to couple the pitch controloutputs to a flight director for semiautomatic operation.

A full understanding of the invention, and of its further objects andadvantages and the several unique aspects thereof, will be had from thefollowing description when taken in conjunction with the accompanyingdrawings in which:

FIG. 1 is a functional block diagram of an aircraft autopilot andautothrottle control system showing the present interactive flightcontrol mode logic and display;

FIG. 2 is a front view of the automatic flight control system modecontrol panel including controls and displays for use in an autopilotand autothrottle control system in accordance with a preferredembodiment of the present invention; and,

FIG. 3 is an enlarged fragmentary view of two parts of the automaticflight control system mode control panel of FIG. 2 including inschematic form the present interactive vertical path control systemproviding integrated pitch and thrust control.

Turning now to FIG. 1, a complete pitch axis 64 and engine controlsystem 84 is seen including autopilot 16 and autothrottle 18 utilizingstate of the art control laws. Upstream from pitch axis 64 and enginecontrol system 84, mode control circuit 12 utilizes a single set ofpilot selectable controls, viz. mode select, speed, and altitude select(as shown in more detail in the schematic embodiment shown in FIG. 3) tocontrol autopilot system 16 and autothrottle control system 18 of pitchaxis 64 and engine control system 84.

Turning now to automatic flight control system mode control panel 10 ofFIG. 2, it will be readily seen when comparison is made with FIG. 3,that the controls and displays of interest in control of the aircraftare mode select control 20 (shown as but not required to be, a pushbutton type control), speed select control 22, selected speed display24, (parameter) airspeed or Mach select control 26, altitude selectcontrol 28, and selected altitude display 30. In FIG. 2, one channel ofautopilot is required to be engaged in CMD (at 32) or F/D (at 34).Another standard control consideration in deployment of the presentsystem embodiment is that autothrottle 18 (shown in FIG. 1) must bearmed at 37 on FIG. 2 (or the speed reference bug on Mach-AirspeedIndicator used for manual throttle control).

FLIGHT LEVEL CHANGE MODE (FL CH)

The FL CH mode of operation is manually selected by operating FL CH MODEREQUEST CONTROL switch 20 shown in FIG. 3. Altitude logic circuit 40operates to determine the polarity and magnitude of altitude errorsignal 42 (which is the difference at the output of combining circuitmeans 45 between aircraft altitude signal 44 provided by aircraft airdata system 46 and selected altitude signal 48 determined by flight crewmanual selection at altitude select control 28).

If altitude error signal 42 is low valued, i.e. representative of Δh inthe expression: |Δh|>×ft, then AND gates 52, 56 and 58 are inhibited, sothat relays 60, 62, and 66 cannot be energized. Also, if altitude errorsignal 42 is low valued, the selection of the FL CH MODE isautomatically cancelled and mode control circuit 12 reverts to theAltitude Hold mode of operation (a feature not explicitly shown in FIG.3). The value of X is a variable dependent upon altitude capture controllaws and the current conditions e.g. at, or close to, the selectedaltitude; climbing towards the selected altitude; or descending towardsthe selected altitude.

If altitude error signal 42 is greater than X ft, either positive ornegative, AND circuit 52 is turned ON by the logic combination(|Δh|>×ft) of altitude logic circuit 40 AND FL CH MODE REQUESTED,thereby causing relay 60 to be energized. Energizing relay 60 switchesthe output to pitch axis control system 64 from an altitude error (Δh)representative signal 42 to a speed error (ΔIAS or ΔM) representativesignal 47 and the pitch axis will compute output commands to control theaircraft speed to the selected value by means of elevator control. Theselected speed reference can be either an airspeed value or a Machnumber value, and the elevator control can be either direct (through thepitch autopilot servo) or indirect (through manual control inputs inresponse to flight director pitch commands generated by the pitch axiscontrol system.

If the aircraft is below the selected altitude, i.e. Δh>+X, AND circuit56 will also be turned ON which causes relay 62 to be energized andallows thrust error signal 70 (provided by subtraction of signal 72representative of desired thrust from signal 74 representative of actualthrust in combining circuit 76) to be coupled downstream through thenormally open contacts of relay 62.

Desired thrust signal 72 is generated by thrust rating select controlmeans 78 and comprises a forward thrust limit value selected by the crewor alternatively, by an aircraft performance computer. Since relays 62and 66 cannot both be energized simultaneously, relay 66 will bedeenergized and thrust error signal 70 will be coupled on downstreamthrough the normally closed contacts of relay 66 to engine controlsystem 84. The engine control system responds by automatically drivingthe engine controls to capture and maintain the selected forward thrustlimit value. In this condition, the excess engine thrust will cause theaircraft to climb with the aircraft speed controlled through theelevator and hereinabove described.

The aircraft will continue to climb in this manner until it approacheswithin X ft of the selected altitude, at which point AND functioncircuits 52 and 56 are both turned off again which results in relays 60and 62 being deenergized. This action causes the output to pitch axiscontrol system 64 to revert to altitude error signal 42 and control theaircraft to capture and maintain the selected altitude through elevatorcontrol, and the output to engine control system 84 to revert to speederror 47 and control the engine thrust to maintain selected speed 80 setby speed select control 22.

If altitude error signal 42 is of negative polarity, that is, Δh<-X whenthe FL CH mode is selected by climb/descent mode request control 20,i.e. the aircraft is above the selected altitude, a similar sequence ofevents to the preceding will occur in mode control circuit 12 exceptthat, in this case, relay 66 will be energized (by the AND function ofcircuit 58) instead of relay 62. In this condition the output to enginecontrol system 84 will be a command to a predetermined "IDLE" position.

The idle position may be a variable, depending on engine anti-ice ON/OFFetc., and may also be a "one-time" command to allow the flight crew tomake subsequent manual adjustments of the thrust setting. The reductionin thrust will cause the aircraft to descend towards the selectedaltitude with the aircraft speed being controlled to the selected valuethrough elevator control.

Similarly to the climb case, when the aircraft descends to within X feetof the selected altitude, the pitch axis control parameter will revertto altitude error causing the aircraft to capture and maintain theselected altitude through elevator control and the engine control systemcontrol parameter will revert to speed error causing the aircraft tomaintain the selected speed through thrust control.

We claim:
 1. In combination in an aircraft:a pitch axis control system;an engine control system; means for providing a signal representative ofa desired selected reference speed; means for providing logic commandsignals in response to desired climb, descend, or maintain altitudemaneuvers of said aircraft; and, switching means responsive to saidlogic command signals for automatically selecting said pitch axiscontrol system to control the aircraft to said selected reference speedwhen said aircraft is commanded to climb or descend, and forautomatically selecting said engine control system to control theaircraft to said selected reference speed when said aircraft iscommanded to maintain altitude.
 2. In combination in an aircraft:anengine control system; means for providing an altitude error signalrepresentative of the difference between a predetermined altitude andthe actual altitude of said aircraft; means for providing climb ordescend command logic signals in response to said altitude error signalrepresentative of aircraft altitude more than a given distance below orabove respectively said predetermined altitude; and, switching means forproviding said engine control system with an increase thrust command inresponse to said climb command logic signal, and for providing saidengine control system with a decrease thrust command in response to saiddescend command logic signal.
 3. In combination in an aircraft having apitch axis control system and an engine control system:an aircraft airdata system for providing a signal representative of aircraft altitudeand a signal representative of aircraft speed; speed select means forproviding a signal representative of selected speed; combining circuitmeans responsive to said signal representative of aircraft speed andsaid signal representative of selected speed for providing a signalrepresentative of speed error; altitude select control means forproviding a signal representative of selected altitude; combiningcircuit means responsive to said signal representative of altitude andsaid signal representative of selected altitude for providing a signalrepresentative of altitude error; thrust rating select control anddisplay means for providing a signal representative of desired thrust;engine sensor means for providing a signal representative of actualengine thrust; combining circuit means responsive to said signalrepresentative of desired thrust and said signal representative ofactual engine thrust for providing a signal representative of thrusterror; flight level change mode request control means for providing asignal representative of flight level change mode requested; altitudelogic circuit means responsive to said signal representative of altitudeerror for providing first, second, and third signals; a first AND gateresponsive to said signal representative of flight level change moderequested and said second signal for providing a signal representativeof flight level change mode engaged; a first relay means having a firstinput terminal, a second input terminal, and an energizing circuit; saidfirst input terminal of said first relay means responsive to said signalrepresentative of altitude error; said second input terminal of saidfirst relay means responsive to said signal representative of speederror, said output terminal of said first relay means coupled to saidpitch axis control system, and said first relay means energizing circuitresponsive to said signal representative of flight level change modeengaged; a second relay means having a first input terminal, a secondinput terminal, an output terminal, and an energizing circuit; a secondAND gate having a first input terminal, a second input terminal, and anoutput terminal; a third relay means having a first input terminal, asecond input terminal, an output terminal, and an energizing circuit;and a third AND gate having a first input terminal, a second inputterminal, and an output terminal; said first input terminal of saidsecond AND gate responsive to said third signal, said second inputterminal of said second AND gate responsive to said signalrepresentative of flight level change mode requested, and said secondinput terminal of said second AND gate connected to said second inputterminal of said third AND gate; said first input terminal of said thirdAND gate responsive to said first signal, and said output of said thirdAND gate coupled to said energizing circuit of said third relay means;said output terminal of said second AND gate coupled to said energizingcircuit of said second relay means; said first input terminal of saidsecond relay means responsive to said signal representative of speederror signal, said second input terminal of said second relay meansresponsive to said signal representative of thrust error, and saidoutput terminal of said second relay means connected to said first inputterminal of said third relay means; said second input terminal of saidthird relay means responsive to signal representative of command idlecomposition of said aircraft, and said output terminal of said thirdrelay means coupled to said engine control system.
 4. In combination inan aircraft:means for providing a first signal representative ofselected reference altitude; means for providing a second signalrepresentative of the altitude of said aircraft; means for combiningsaid first and second signals for providing an altitude select errorsignal; means responsive to said altitude select error signal forproviding logic signals corresponding to desired climb to or descend toor maintain said selected reference altitude; a pitch axis controlsystem; a thrust axis control system; and, means coupled in said pitchaxis and thrust axis control systems and responsive to said logicsignals for causing said aircraft to climb to, descend to or maintainsaid selected altitude.
 5. In combination in an aircraft:a pitch axiscontrol system; a thrust axis control system; means for providing afirst signal representative of a selected reference altitude; means forproviding a second signal representative of the altitude of saidaircraft; means for combining said first and second signals to producean altitude select error signal; means responsive to said altitudeselect error signal for providing logic signals corresponding to adesired climb, descend, or maintain altitude maneuver of said aircraft;means for providing a third signal representative of a selectedreference airspeed; means for providing a fourth signal representativeof the airspeed of said aircraft; means for combining said third signaland said fourth signal to provide an airspeed error signal; switchingmeans responsive to said logic signals for transferring control ofairspeed of said aircraft between said pitch axis control system andsaid thrust axis control system; and means connected between said pitchaxis control system and said thrust axis control system and responsiveto said airspeed error signal for airspeed control of said aircraft. 6.In combination in an aircraft:a pitch axis control system; an enginecontrol system; means for providing an altitude error signalrepresentative of the difference between a desired selected altitude andthe actual altitude of the aircraft; means for providing a speed errorsignal representative of the difference between a desired selected speedand the actual speed of the aircraft; means for providing an increasethrust command signal; means for providing a decrease thrust commandsignal; logic circuit means responsive to said altitude error signal forproviding a descend command logic signal when said actual altitude ismore than a predetermined distance above said selected altitude, a climbcommand logic signal when said actual altitude is more than apredetermined distance below said selected altitude, and a maintainaltitude command logic signal when said actual altitude is within apredetermined distance of said selected altitude; switching meansresponsive to said logic signals for coupling said speed error or saidaltitude error signal to said pitch axis control system and furthercoupling said speed error, or said increase thrust command signal, orsaid decrease thrust command signal to said engine control system forenabling said pitch axis control system to control said selected speedunder both climb and descend command logic conditions and to control theaircraft to said selected altitude for the maintaining altitude commandlogic condition; and said engine control system providing control tosaid increase thrust command signal for said climb command logiccondition, to said decrease thrust command signal for said descendcommand logic condition, and to said selected speed for said maintainaltitude command logic condition.
 7. The invention according to claim 6further comprising a single mode select means for enabling said logiccircuit means and said switching means.
 8. The invention according toclaim 6 further comprising a single control device for providing adesired selected speed for utilization when either said pitch axiscontrol system or said engine control system is controlling the speed ofsaid aircraft.